DEXTRALITY AND SINISTRALITY. 367 



times. That this sign-language of primitive man was dextral is not 

 to be questioned, as about d8 per cent, of babies are now clearly right- 

 handed before they are one year old. The protection of the heart by 

 the shield would constitute sufficient reason for the institution of 

 dextrality in counting and sign-making, and custom and uniformity 

 of habit especially in early times, would result in almost a universality. 

 But not an absolute one, for one or two per cent, are now sinistral. 

 And the Bible story of the Benjamite tribe illustrates how the habit 

 would not be absolute. There is in all this one noteworthy neurologic 

 fact : In view of the long continuance and vast preponderance of dex- 

 trality it seems strange that the brain preserves all the preformed 

 mechanisms, plastic and ready to make a sinistral child, and the out- 

 working of sinistrality is as prompt, the result as dextrous, as if dex- 

 trality had been chosen. The wonder at this is, however, lessened when 

 one notes that all the functions of completed dextrality are at the 

 same time and in the same person now possible to the sinistral; there 

 is a mere difference in the degree not in the kind of expertness. Besides 

 this a number of left-handed acts in the dextral, e. g., those of the 

 violinist, gunner, etc., are far more expertly and finely coordinated 

 than those of the right, etc.* 



If the foregoing explanation of the origin and perpetuation of 

 dextrality is adequate, it remains to explain the origin of sinistrality. 

 Why are there about two out of a hundred naturally left-eyed and left- 

 handed ? Fundamentally, of course, because the speech-center is located 

 in the right cerebral hemisphere, and the contributing and executing 

 centers of vision and motion act in better unity if they are in close 

 connection and contiguity than if connected by long commissural fibers 

 to and from the opposite sides of the brain. The dextrocerebrality 

 of two per cent, of sinistral exceptions to the usual law appears explain- 

 able, perhaps in part by persistence of original sinistral types, but 

 more certainly they are due to accident, injury, disease, etc., of dextral 

 organs, in the young of our ancestors. Especially in savage life would 

 these accidents be more numerous than now. The loss of even one 

 dextral finger might compel the education of the undeveloped speech 

 center on the right side. Injury to the right hand and arm, even of 

 the right foot or leg would do the same. Deafness of the right ear 

 would compel a turning of the left ear forward and might work out 

 complete sinistrality. But more important than all these causes com- 

 bined would be the more frequent greater ametropia, amblyopia, dis- 



* There is thus no danger and no need of a greater weight of the half brain 

 initiating dextrality in the dextral, and all the discussion and labor of com- 

 parative weighing the two halves is relatively useless. Moreover the cerebral 

 mechanisms must be equally perfect even if not equally exercised. Taken in the 

 average the two sets of organs, central and peripheral, do about the same 

 amount of work. 



